Wireless communication systems are used for a variety of applications, such as identification, tracking, radar, data communications, voice communications, and video communications. In radio frequency (RF) digital radio systems, a terminal unit communicates with other terminal units using modulated radio signals. The terminal units include digital receivers which demodulate the incoming digitally modulated RF carrier to reproduce the data message being transmitted.
A software radio receiver system is a baseband receiver architecture and implementation in which all of the baseband receiver functions are performed digitally, typically utilizing a digital signal processor or a general purpose processor, in which the processor executes program instructions to perform the baseband processing functions. As such, software radio takes the received radio signal, typically after having been demodulated to an intermediate frequency (IF), and recovers the channel symbol bits. Current software radio technology is limited in terms of the channel symbol rates which can be digitally demodulated. A digital signal processor or general purpose processor is basically a serial computational device, thus limiting the processor's computational speed. Thus, wideband technology, such as code division multiple access (CDMA) systems with symbol rates near to or exceeding one megasymbol per second cannot currently be practically implemented in a traditional software radio approach. In summary, software radio techniques are limited due to the processing capabilities; they can either support narrowband technology, or must be coupled with other techniques in order to support wideband technology.
Current technology also supports multiple mode terminals. For example, cellular telephony supports dual or triple mode terminal units, where each mode communicates at a different frequency and utilizing a different protocol mode. As an example, a cellular telephone could support the analog advanced mobile phone system (AMPS) 30 kHz bandwidth air interface standard, and could also support a time division multiple access (TDMA) air interface standard based upon having multiple time slots within the same 30 kHz channel. However, in technology used to build dual-mode radio communication systems, the protocol mode is determined at call set-up time, and remains fixed for the duration of that call. Although some of the hardware can be reused between these modes, a dual-mode cellular telephone typically utilizes duplicative or redundant hardware, such as the RF and intermediate frequency (IF) filters, with the software digital receiver implementing the baseband or IF-to-baseband processing functions. As discussed above, software radios are limited to narrowband modulation processing (typically less than 100 ksamples/sec), with additional hardware needed to perform wideband modulation processing (typically greater than 100 ksamples/sec).
Beyond the current limitations of software radio technology, there are many impairments in the RF channel, such as shadowing due to the geographic terrain, Rayleigh fading due to the constructive and destructive addition of multipath signals, and interference from other radios, which disrupt the successful transfer of data between the terminal units and to reduce the throughput of the information being transferred. Additionally, these RF impairments are time varying. To overcome RF impairments, complex transmitter/receiver schemes and devices have been developed, such as convolutional coders and adaptive antenna arrays. These complex schemes are designed based upon the worst case RF impairments of the RF channel to achieve certain quality of service measures which can be measured by various parameters, such as bit error rate (BER), packet error rate and latency or delay in subsequently delivering information previously corrupted. These schemes reduce the information capacity of the communication system to counter the expected RF impairments. However, if an RF channel is in a relatively uncorrupted state and not suffering from any impairments, current schemes do not take advantage of the full information capacity of the available bandwidth because bandwidth is still being used to counter the RF impairments.